Strong Signs of Quantum Spin Observed in Ytterbium Compound

Inside a new exotic crystal cooled to near absolute zero, physicist Martin Mourigal has observed strong indications of quantum entanglement, a theory so weird Albert Einstein lampooned it as “spooky action at a distance.” Entanglement occurs when two particles, such as electrons, become intimately linked to one another even when separated by many miles. Actions applied to one particle then instantaneously impact the other.

Researchers have proven entanglement in previous experiments, but scientists like Mourigal, an experimental physicist at the Georgia Institute of Technology, have taken it much further. Mourigal and his team examined YbMgGaO4, a ytterbium compound the team says is likely brimming with observable “spooky” connections.

The entanglement Mourigal’s team observed made a system of electrons a quantum spin “liquid.” He doesn’t use the term in the usual sense, as in water, but rather to describe the collective nature of electrons’ spins in the crystal. “In a spin liquid, the directions of the spins are not tidily aligned, but frenzied, although the spins are interconnected, whereas in a spin solid the spin directions have a neat organization,” Mourigal says.

If the discovery stands, it could open a door to hundreds of yet unknown quantum spin liquid materials that physicists say must exist according to theory and mathematical equations. In the distant future, new quantum materials could become, by today’s standards, virtual sorcerer’s stones in quantum computing engineers’ hands.

Scientists in China first synthesized the ytterbium crystal a year ago. The Chinese government has invested heavily in hopes of creating synthetic quantum materials with novel properties. It appears the research concept may have now succeeded, according to Mourigal. “Imagine a state of matter where this entanglement doesn’t involve two electrons but involves, three, five, 10 or 10 billion particles all in the same system,” he says. “You can create a very, very exotic state of matter based on the fact that all these particles are entangled with each other. There are no individual particles anymore, but one huge electron ensemble acting collectively.”

One of the only previously observed apparent quantum spin liquids occurs in a natural crystal called herbertsmithite, an emerald green stone found in 1972 in a mine in Chile. Researchers named it after mineralogist Herbert Smith, who died nearly 20 years before the discovery. Scientists observed its apparent spin liquid nature in 2012 after Massachusetts Institute of Technology researchers succeeded at reproducing a purified piece of the crystal in their lab.

Physicists from the University of Tennessee succeeded in replicating the original ytterbium crystal, and Mourigal examined it at Oak Ridge National Laboratory (ORNL) (CSA CSM), cooling it down to a temperature of -273.09°C. The cooling slowed the natural motion of the atoms to a near stop, allowing Mourigal’s team to observe the electron spins’ dance around the ytterbium (Yb) atoms in the YbMgGaO4 crystal. The team used a powerful superconducting magnet to line the spins up in an orderly fashion to create a starting point for observations.

“Then we removed the magnetic field, and let them go back to their special kind of wiggling,” says Mourigal. His team carried out the observations at the ORNL Spallation Neutron Source, a US Department of Energy Office of Science User Facility that allowed the scientists to watch the concert of electrons’ spins by bombarding them with neutrons.

When one electron flips its spin, researchers generally expect it to create a neat chain reaction, resulting in a wave going through the crystal. The wave of electron spins flipping in sequence might look something like fans at a football game standing and sitting back down to make a wave go around the stadium.

But something odd happened here, according to Mourigal. “This jumbly kind of spin wave broke down into many other waves, because everything is collective, everything is entangled. It was a continuum of excitations, but breaking down across many electrons at once.” He says it was qualitatively similar to what was observed using the same technique on herbertsmithite.

To authenticate the observations made by Mourigal’s team, theoretical physicists will have to crunch the data with methods that, in part, rely on topology, a focus of the 2016 Nobel Prize in Physics. Mourigal thinks chances are they will pass muster. “At first glance, this material is screaming, ‘I’m a quantum spin liquid,'” he says.

But the material must first undergo a years-long battery of stringent mathematical tests where theoretical physicists will wrap the data around a mathematical “donut” to confirm whether or not it is a quantum spin liquid. “That’s meant seriously,” Mourigal says. “As a mathematical mental exercise, they virtually spread the spin liquid around a donut shape, and the way it responds to being on a donut tells you something about the nature of that spin liquid.”